Electrostatic precipitators can be used for dust collection in sintering plants which can have supply bins and handling means for iron ore, lime, coke breeze and returns, a driven revolving sintering belt provided with a plurality of suction boxes disposed under the belt, an exhaust fan for withdrawing exhaust gases from the sintering belt, an electrostatic precipitator preceding the exhaust fan, sieving means and an exhaust fan that is preceded by an electrostatic precipitator for dedusting the room. A fine-grained solid conditioning agent can be supplied to the exhaust gas stream to improve the separating rate of the electrostatic precipitator or precipitators.
It has long been known that exhaust gases may be conditioned to improve the separation rate of electrostatic precipitators.
Fine-grained solids as conditioning agents have, however, been given different evaluations aand have not played a significant role in practice thus far.
In accordance with British Patent Specification 559,532 of 1942, non-conductive dusts can be removed from gases or air by electrostatic separation if so-called powdered fuel grits are admixed with the gases or the air.
Such powdered fuel grits consist of the solid particles which are entrained by the exhaust gas from pulverized coal-fired boiler furnaces and are separated in electrostatic precipitators. They constitute a flue ash, which may contain 3 to 70% unburned coal (lines 68 to 82 on page 3 and claim 1).
"Journal of the Institute of Fuel" (May 1963) contains on pages 184 to 197 a report on the influence which is exerted by the properties of coal and of the combustion of the efficiency of electrostatic dust collectors. It has been stated there that the carbon content does not have a strong influence on the separation rate, but the influence of the particle size may be significant. If coal and oil are burned at the same time, an incomplete combustion of the oil may result in a formation of fine soot particles, which will deposit as a layer on the dust particles so that their electrical resistance is strongly reduced and their separation in the electrostatic precipitator is strongly promoted.
On the other hand, unburned matter from the coal will be relatively coarse-grained and will not have a measurable influence on the separation rate (penultimate and last paragraph on page 195, first paragraph on page 196).
"Staub", 25 (1965) contains on pages 402 to 409 a report on the problems arising in connection with electric dedusting. It is stated there that particles having a high electrical conductivity (coke, metal) will adversely affect the separation of dust in an electrostatic precipitator and that a higher degree of separation can be achieved in much smaller electrostatic precipitators if the coal is completely burned and care is taken that coke particles will not enter the electrostatic precipitator (last paragraph on page 408, first paragraph on page 409, Abstract).
In his book "Entstaubung industrieller Gase mit Elektrofilter" (VEB Verlag Leipzig 1969), H. J. WHITE states on page 275: "Very fine particles, such as soot, tend to deposit on insulating particles and to surround the same with a conductive layer. For this reason, they will become effective even in small amounts of only a few percent of the total mass of the dust. On the other hand, coke particles are relatively coarse when compared with the soot and are required in a content of 10 to 20% by mass."
Similar remarks have been made by the author of that book in "Journal of the Air Pollution Control Association", Vol. 34, No. 4 (April 1974) on pages 314 to 338, particularly in the last paragraph of the left-hand column and the first paragraph of the right-hand column on page 330.
The book "Elektrostatic Precipitators" (1982) by Jaraslav Bohm contains the summary statement that conductive coarser particles, such as unburned coke, in flue ash cannot effect an improvement of the separation rate but by a disturbance of the electric field may even result in a deterioration.
It has been stated there that they may improve the separation rate only when present in relatively large proportions (10 to 20%). On the other hand, it has been states there that soot in a content of even less than 10% may strongly decrease the resistance of the dust. It is apparent that the publications mentioned above do not furnish a clear teaching for technical action to a person who desires to improve by a conditioning action, the separation rate of an electrostatic precipitator which is unsatisfactory in operation due to an excessively high dust resistance in cases in which the dust contains potassium chloride so that SO.sub.3, which is known per se and probably is the most effective conditioning agent, cannot be used for the reason.
It will be understood that as in all conditioning processes, the expenditure must not exceed the advantages which are afforded. Coke and soot are combustible substances, which will not carelessly be discharged in appreciable quantities together with the dust if the improvement of the separation rate can be effected more economically by different methods.
In sintering plants, it is necessary to collect dust from the exhaust gases from the sintering belt and from the room air. The gases to be dedusted become available at high rates and have high contents of dust having a low electrical conductivity so that conditioning agents are required at a correspondingly high rate.
It is an object of the invention to provide an economically acceptable conditioning without the use of SO.sub.3 for this purpose, especially in an iron sintering process.